Following the completion of circuit boards provided with conductor paths consisting of copper or copper alloys, residues and impurities resultant from the production give rise to difficulties regarding the solderability of the conductor paths which are further increased as a result of a long storage of the circuit boards. Experience has shown that the best remedy regarding soldering faults is provided by a careful and suitable cleansing of the circuit boards and the subsequent application of a soldering agent. To this end, the surfaces of the conductor paths can be freed of impurities mechanically by sand-blasting, scouring, grinding, buffing, brushing and even by erasing with a special rubber. However, in these mechanical cleansing processes, it is difficult to control the extent of the removal of metal from the surface of the conductor paths. In addition, particles of the grinding agents which are employed and also of the abraded material can settle at points to which it is difficult to gain access and can impair a subsequent soldering.
For this reason generally, chemical cleansing processes are given preference. Thus, the U.S. Pat. No. 3,020,175 discloses a chemical cleansing process for electric circuit boards in which the surfaces of the conductor paths are freed of dirt and oxides in a first acid oxidizing solution, for example, a mixture of chromic acid and sulphuric acid. Subsequently, the surface of the conductor paths is activated, i.e. brought into a condition of good solderability, in a second solution, for example, dilute hydrochloric acid. In this second solution salt films, such as, for example, chromate films which have formed during the treatment in the first solution, are completely removed. The improvement in solderability which is achieved with this chemical cleansing process reduces considerably, however, with increasing length of storage of circuit boards after such a treatment. Furthermore, the baths employed in chemical cleansing processes not only require expensive servicing, but also are generally very agressive and complicated to prepare in respect of environmental waste disposal technology.